2014
SZ3085 : Alum Bay Unconformity
taken 10 years ago, 3 km from Totland, Isle of Wight, England
Alum Bay Unconformity
An unconformity is a gap in the geological record. Unconformities occur when a succession of sedimentation stops, perhaps because of a rise in the land relative to the sea, so deposition ceases and erosion may occur. Thousands or millions of years later, another change occurs which leads to the resumption of deposition of sediment on the much older surface. On the right here is the Upper Chalk, with a layer of fine brown mudstone originally deposited on top of the chalk, but also infilling the sinkholes, and to the left are the Reading Beds, the first unit of the much younger Tertiary sediments that form the famous coloured cliffs. The unconformity is the plane where the soft mudflow in the foreground meets the base of the cliff.
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